Fuel Pressure Problem.

[StealthX]

Hey guys. A little issue came about when my car went in to get tuned the other day.

It's a GT2860rs turbo, walbro 255lph pump, WRX 440cc injectors and an EMS standalone engine computer.

Now from stock, the X-Trail runs a returnless fuel set-up. i.e. no FPR, no return lines. The fuel pressure is governed at the tank, gosh darn knows how... All i know is it has about 52psi of fuel pressure in the rail, and at this 13psi of boost, its dropping.

Here's the dyno graph of power vs AFR:

(remember that power figure is at 4 wheels, not just 2!)

I'm starting to go lean at 13psi as you can see...

I'm not sure if my injectors are maxing out, but I wouldn't imagine they would cause this type of problem, would they?

WHATS GOING ON!??

p.s. the reason for my wacky boost curve is the dump, its getting sorted soon...

[zebra]

You might need to setup a return line and FPR.

How big are the fuel lines?

[Crans]

the OEM fuel pressure regulation system probably isn't designed for boosted applications. Might be a good idea to rip it out set up the fuel pump to supply the fuel rail and put a FRP on the fuel return as per your normal Skyline setup

[StealthX]

Yeah that's what i thought. I just went for a drive with the laptop hooked up looking at injector shit.

Under full boost, they never got close to maxing out, i didnt see them go over 70% even at top revs.

@ zebra, i dont really know how big they are mate...

[zebra]

With out a rising rate / boost referenced fuel reg the +ve manifold pressure forces back against the incoming fuel.

so although the injectors are seeing 52psi under vacuum, with 13psi in the intake they really only have 39psi of effective pressure.

which no doubt is causing the lean off.

[Harey]

Quote from here: http://www.aa1car.com/library/returnless_efi.htm

" A returnless fuel injection system, by comparison, manages fuel pressure a little differently. Instead of using a spring-loaded vacuum diaphragm in the regulator to change fuel delivery when throttle opening and intake vacuum change, the regulator in a returnless system operates at a constant pressure. The older return-type systems need to vary fuel pressure to maintain the same pressure differential across the injectors when intake vacuum drops. When vacuum drops, the regulator increases pressure to compensate. But in a returnless system, this isn't necessary because the line pressure is always the same.

So how does the system compensate for changes in engine load and vacuum? A returnless system uses the Powertrain Control Module (PCM) to regulate fuel delivery. A fuel pressure sensor mounted on the supply rail allows the PCM to monitor fuel pressure. When pressure in the supply rail drops as engine load or speed increase, the PCM compensates by increasing injector duration (on time) and/or the operating speed of the fuel pump."

1psi of boost basically reduces the fuel pressure by 1psi. Therefore, if the fuel pressure going in is 52psi, under 13psi of boost, the fuel rail pressure will only be 39psi! The only way to combat this is for the ecu to get more out of the injectors, however the injectors can only flow so much fuel at 39psi. I am assuming your tuner cant put in any more fuel, therefore if you dont want to change the fuel system design I believe you need bigger injectors.

Edited April 29, 2010 by Harey

[Harey]

Under full boost, they never got close to maxing out, i didnt see them go over 70% even at top revs.

That is very strange, I would be quizzing your tuner and saying why dont you put more fuel in??

As said above, your ecu needs to compensate for the lack of effective fuel pressure. I ran a turbocharged car with 43psi of effective fuel pressure, so I dont see why you cant on 39psi. You just need your ecu and injectors to compensate.

Edited April 29, 2010 by Harey

[StealthX]

That is very strange, I would be quizzing your tuner and saying why dont you put more fuel in??

As said above, your ecu needs to compensate for the lack of effective fuel pressure. I ran a turbocharged car with 43psi of effective fuel pressure, so I dont see why you cant on 39psi. You just need your ecu and injectors to compensate.

Yeah Andrew did, you can see the A:FR drop rich just before it starts to go lean, so he tried. I dunno about just then if i read the duty cycle correctly, it was hard to read the computer screen, wrestle with the steering wheel from the torque steer/wheel spin AND watch the road at the same time...

Thanks for the help! looks like i might just fix this boost curve and run a flat 10psi, then avoid touching the fuel system at all, cos its a prick of a job on my car entire intake manifold needs to come off just to get to the injectors!

[Harey]

Dont give up yet

http://autospeed.co.nz/cms/title_Modifying...36/article.html

Are you running the stock fuel pump? Have you ensured your fuel pump is getting max voltage?

Edited April 29, 2010 by Harey

[R31Nismoid]

the OEM fuel pressure regulation system probably isn't designed for boosted applications. Might be a good idea to rip it out set up the fuel pump to supply the fuel rail and put a FRP on the fuel return as per your normal Skyline setup

Exactly what I'd do

Rip the shit out from the tank end, run the pump... fit a FPR up the front and run a return line back and problem solved.

[Harey]

Exactly what I'd do

Rip the shit out from the tank end, run the pump... fit a FPR up the front and run a return line back and problem solved.

Yes that would be the best solution, but its far from simple.

39psi can be enough fuel pressure if the injectors can flow enough fuel (also if the fuel pump can flow enough).

Increasing the voltage to the stock fuel pump could also be enough. It doesnt look like it needs a lot more fuel. Whether you do this by running dedicated power cables from the battery or you use a voltage amplifier.

Edited April 29, 2010 by Harey

[R31Nismoid]

One assumes the pump is wired correctly

[StealthX]

Thatt was a great article Jeff, thanks!

For now I might focus on fixing that boost curve... 13psi is too much for what I wanted, so I think I'll address that issue rather than the fuel system... The guys in the US who mod these QR25 engines use return setups with fprs so I'll go with that sorta setup of I need more fuel. At the moment Im pretty close to the limit of the stock block and the chassis, I desperately need some suspension mods and a bit more rubber (I've only got 215s at the moment) for some better handling/traction.

I'm armed with all these options now, so that's great!

Thanks guys!

[StealthX]

One assumes the pump is wired correctly

The walbro dropped in perfectly, needed no wiring/plumbing mods at all...

[femno]

Not having enough fuel pressure in the rail seems dodgy.

If I wanted to feel comfortable driving the car hard and wanted to know the setup was solid, id do things properly.

440 injectors for 220HP is heaps. I think changing injectors out is masking the real problem of a weak fuel system.

It aint good to run shit on the limit, especially fuel.

Braise a fitting on the end of the rail, run a Sard reg, and run an extra line to the rear for the return. Also hard wire the pump. Ditch the Walbro and go 040. Piss easy.

[R31Nismoid]

The walbro dropped in perfectly, needed no wiring/plumbing mods at all...

So basically you probably haven't wired it correctly...

You need to ensure the pump has the correct voltage it requires!

Bolt-in does not mean "work correctly (especially on a N/A previously setup)...

[StealthX]

So basically you probably haven't wired it correctly...

You need to ensure the pump has the correct voltage it requires!

Bolt-in does not mean "work correctly (especially on a N/A previously setup)...

I see where you're coming from, cos in theory, the bigger pump should need different voltages!

According to the guys in the US with the same motor, the walbro is a direct swap. uses the same voltage as the stock one as far as i know.

Hundreds of guys have done turbo setups, similar to mine and there's not one mention of altering the wiring at all, that's why i chose the 'bro over a bosch. They are a drop in and (usually) good for upwards of 400whp.

Just talked to Andrew on the phone and he said the pump and/or injectors are fine for well over the power im making, so im leaning (pardon the pun) towards the stock in-tank regulator doing weird things and not having enough pressure to support the boost.

Either way its not a huuge problem, its only starting to lean out on 13psi and 5500 revs, and on the street in the last 2 days, i havnt seen 13psi yet. The load on the dyno is substantially more than on the street in 1st 2nd and 3rd gears so hopefully i wont have much of an issue to deal with.

[R31Nismoid]

Testing the voltage is such a simple thing to check mate, to simply rule out that it's not an issue I'm not sure why it wasn't done?

Just because other people drop in and it works is great, but doesn't automatically mean that voltage isn't your problem if you've not checked it.

[StealthX]

Ok sure I'll have a look. what should the voltage be? Won't it be hard to measure?

I was under the impression that said voltage changes with fuel needs in a returnlessn system, so it would be hard to be in the back seat with a voltmeter, when I need to vary the engine conditions to check!

Or am I mistaken?

[rob82]

In the 30 odd so of boosted Gen3 and 4 I've tuned I've never had an issue up to 350rwkw and 10Psi of boost using the std 1 one way fuel system - they do however have injector flow scaliing vs injector pressure drop. There is no reason to swap to a return system you have an aftermarket ECU - just tune around its not like the ems has any brains anyway. There should be no reason to assume fueling wont be consistant.

The things I would be checking is whether or there is a fuel pump controller. It maybe PWM if the std ecu is still controlling the fuel pump side of things.

Edited April 29, 2010 by rob82